Multi-dimensional microwave sensing using graphene waveguides

This paper presents an electrolytically gated broad-band microwave sensor where atomically-thin graphene layers are integrated into coplanar waveguides and coupled with mi-crofluidic channels. The interaction between a solution under test and the graphene surface causes material and concentration-specific modifications of graphene's DC and AC conductivity. Moreover, wave propagation in the waveguide is modified by the dielectric properties of materials in its close proximity via the fringe field, resulting in a combined sensing mechanism leading to an enhanced S-parameter response compared to metallic microwave sensors. The possibility of further controlling the graphene conductivity via an electrolytic gate enables a new, multi-dimensional approach merging chemical field-effect sensing and microwave measurement methods. By controlling and synchronising frequency sweeps, electrochemical gating and liquid flow in the microfluidic channel, we generate multi-dimensional datasets that enable a thorough investigation of the solution under study. As proof of concept, we functionalise the graphene surface in order to identify specific single-stranded DNA sequences dispersed in phosphate buffered saline solution. We achieve a limit of detection concentration of ~ 1 attomole per litre for a perfect match DNA strand and a sensitivity of ~3 dB/decade for sub-pM concentrations. These results show that our devices represent a new and accurate metrological tool for chemical and biological sensing.